This invention relates to a method of treating a sulphide mineral or a mixed sulphide and oxide mineral for the recovery of metal content. The invention is described hereinafter with particular reference to the recovery of copper. This is exemplary only, and not limiting, for principles of the invention can be used for the recovery of other base metals such as nickel from nickel sulphide minerals e.g. pentlandite and millerite, and zinc from zinc sulphide minerals.
US 2008/0026450 describes a method of recovering copper from copper sulphide ores containing chalcopyrite in which leaching is carried out by means of a chloride ion-resistant sulphur oxidising bacterium in a leaching solution which has a chloride ion concentration of from 6 g/l to 18 g/l and a pH in the range 1.6 to 2.5.
It is suggested that, with this method, the copper leaching rate is promoted by adjusting the chloride ion concentration of the leaching solution. Further, as sulphur is converted to sulphuric acid it becomes possible to prevent a significant reduction in the leaching rate which is otherwise caused by sulphur coating surfaces of the copper sulphide ore.
A similar approach to the aforegoing but restricted to the use of a specific microorganism, is disclosed in US2007/0202584.
An aspect on which the aforementioned patent applications are silent, and which is largely ignored in the prior art, is that the presence of free copper in the form of Cu2 is generally toxic to microorganisms which are used in bioleaching processes. The toxic effect of copper on bioleaching related organisms is especially severe in the presence of chloride ions. This reduces the efficiency of metal recovery.
AU 2002254782 describes a method of bioleaching a sulphide mineral in a solution with a chloride content in excess of 5000 ppm and at a temperature in excess of 10° C. using at least one bacterium selected from those deposited under the accession numbers DSM14175 and DSM14174.
Despite this disclosure the limited tolerance to salt of the bacteria means that in many instances large volumes of fresh water are still required in bioleaching mineral processing operations. Also, at relatively low temperatures, e.g. up to 45° C., the leaching of chalcopyrite is generally inefficient.
The invention is concerned with a bioleaching method which exhibits increased efficiency and which can be implemented under high chloride ion content conditions.